Electrodeionization is a recognized means for achieving high levels of desalination of aqueous liquids. The process involves an ion exchange material, such as an ion exchange resin, which is in a feed or ion depleting compartment formed in a gasket positioned between anion and cation exchange membranes.
The invention may find many uses such as processing dextrose, sucrose, and fructose, for example. An aqueous feed stream that needs to be desalted is circulated in the depleting compartment.
The electrodeionization process is detailed in a number of U.S. patents. The more important and recently issued U.S. Pat. Nos. are: 4,066,375; 4,203,976; 4,249,422; 4,465,573; 4,632,745; 4,727,929; 4,871,431; 4,925,541; 4,931,160; 4,956,071; 4,969,983; 5,066,375; 5,116,509; 5,120,416; 5,203,976; 5,292,422; 5,308,466 and 5,316,637. The gaskets or chambers used in the ion depleting compartments (i.e., the compartments containing ion exchange material), disclosed in these patents are of various designs.
U.S. Pat. No. 4,465,573 describes a gasket containing narrow "tortuous" flow paths that are filled with ion-exchange resins. These paths are formed by a plurality of parallel "ribs" extending from the stream inlet to the outlet. The water stream that needs to be deionized (purified) is passed through these paths. The gasket design of the patent is complex, requiring a careful meshing of the gasket components for the feed and concentrate streams, and will likely have a high pressure drop.
U.S. Pat. Nos. 4,632,745; 4,747,929; 4,925,541; 4,931,160 and 4,956,071 (assigned to the Millipore Corporation) describe a gasket, apparatus and process wherein the spacer construction for the feed (also termed the depletion or the ion depletion) compartment comprises a feed inlet and outlet with a distribution manifold in communication with an inlet and a central section. The distribution manifold feeds the central section. The central section has a thickness which is the same as the thickness of the gasket and is defined by a plurality of ribs extending along the length of the central section to form subcompartments having a width defined by the distance between the two adjacent ribs or by the distance between a rib and an adjacent longitudinal edge section of the gasket. Also, there are fingers extending from the manifold to the inlet of the subcompartments in the central section. An ion exchange material fills the subcompartments, the feed and, optionally, the concentration compartments.
The subcompartments are relatively narrow, preferably between 0.5 and 1.5 inches. To ensure good sealing, the ion exchange membranes are glued or bonded to the feed compartment gasket. The design is fairly complex and requires a significantly large amount of membrane area sealed to the gasket. Therefore, this sealed area is not available for ion removal. The use of multiple parallel subcompartments improves the rigidity of the gasket and the containment of the ion exchange resin material; however, it requires large fluid distribution areas and can result in fluid distribution problems.
U.S. Pat. Nos. 4,066,375; 4,203,976; 5,120,416; (assigned to Ionics, Inc.) deal with a gasket design and with electrodeionization apparatus that enables an introduction and removal of ion exchange resins and other particulates from assembled deionization stacks. The design either requires a third set of relatively open inlet manifolds to permit an introduction and removal of said resins and particles or requires one of the other two (i.e., inlet or outlet) channels to permit the entry and exit of the particles. To provide a good sealing between the feed and the concentration compartments, particularly at the third inlet manifolds, these membranes have to be sufficiently immobilized and must not be prone to swelling/wrinkling. The introduction of the third set of ports introduces additional sealing problems as well as a reduction in the useful membrane area available for the actual desalting operation.
While the ability to fill and empty the particulates from the electrodeionization stack after the assembly of the gaskets and membranes is an advantage, one still has to disassemble the stacks if there are problems related to membrane fouling, leakage or breakage. Additionally, a proper packing of the desalting compartment is not assured, particularly for large stacks and when dealing with multiple flow paths. The design also becomes superfluous, if ion exchange materials in the form of a felt or fabric are deployed instead of resin beads.
U.S. Pat. No. 4,249,422 (assigned to IP Holdings) discloses an electrodeionization apparatus having an improved sealing between the membranes and the gaskets. The apparatus employs a plurality of disk shaped diluting and concentrating cell pairs. This apparatus facilitates filling the individual cells with ion exchange resins. While offering some improvements over the earlier designs, it is still rather complex.
Therefore, a need exists for an improved electrodeionization gasket and associated apparatus that enables a greater utilization of the ion exchange membranes and that can use high performance membranes, i.e., thin, higher selectivity, low electrical resistance or unreinforced type membranes, that may be prone to swelling/wrinkling. These membranes must be adequately supported in the central area and sealed against the feed and concentration compartment gaskets.
A need also exists for a gasket that assures good fluid flow and electrical current distribution and that has a low overall pressure drop for fluid flow. Also, the gasket should contain the ion exchange material without excessive migration of resin beads during extended periods of operation.
A further need exists for a gasket that is simple in construction, that can be built in a large size (exceeding 1 m.sup.2 in overall area), that can efficiently use the ion exchange material in either the resin bead or felt/cloth form, and that is easy to assemble/disassemble into an electrodeionization apparatus. The apparatus should be capable of modular assembly.